The present invention generally relates to memory circuit write-in systems, and more particularly to a memory circuit write-in system which writes information data within a digital signal which is transmitted including a time base fluctuation component (jitter component) into a memory circuit, with a write-in period in a range of two times within a transmission period corresponding to one frame (one block) of the digital signal.
Generally, systems for recording and reproducing information signals onto and from a rotary recording medium (hereinafter simply referred to as a disc) have been reduced to practice. According to such systems, an analog information signal such as a video signal and an audio signal is subjected to digital pulse modulation such as pulse code modulation (PCM), and the digital information signal is recorded onto the disc as variations in geometrical configuration consisting of rows of intermittent pits. The recorded signal is reproduced from the disc by detecting the variations in the intensity of light reflected from the disc or variations in electrostatic capacitance between the disc and a reproducing element. When reproducing the recorded signal from the disc in such systems, the digital signal reproduced from the disc generally includes a time base fluctuation component (jitter component) due to irregular rotation of the disc and the like.
On the other hand, if errors are introduced in the data upon transmission of the above digital signal, it becomes necessary to correct the error and restore the data. Hence, one frame (one block) of the above digital signal is constituted by a time-sequentially multiplexed signal comprising error correcting codes, error detecting codes, and synchronizing signals for indicating beginnings of the signals, in addition to data words which are divided into predetermined sections. Thus, the digital signal is recorded and reproduced in terms of such frames. Accordingly, it becomes necessary to provide a memory circuit in the reproducing system, in order to store the data words and the error correcting codes. Because the digital signal is reproduced including the above time base fluctuation component, even if the minimum period of fluctuation in the time base fluctuation component is sufficiently large compared to the period of one frame of the digital signal (the period of one frame of the digital signal is generally equal to an integral multiple of the reciprocal of the sampling frequency, and this period of one frame will hereinafter be referred to as "frame period"), the write-in frame period with which the write-in is carried out with respect to the memory circuit and the read-out frame period with which the read-out is carried out with respect to the memory circuit are not in synchronism.
Conventionally, as a method of writing the data words and the error correcting codes within the digital signal which includes the above time base fluctuation component into the memory circuit, there was a method according to which a number of flags corresponding to the number of data words and error correcting codes within one frame were set, and the write-in was carried out in accordance with the time base fluctuation component by cancelling the flags one by one every time the data word or the error correcting code is written into memory elements from one of a plurality of slave latches which are provided. However, according to this conventional method, it was impossible to write the data words and the error correcting codes within one frame together at a single address in the memory circuit, and the write-in of these data words and error correcting codes within the one frame had to be carried out in parts. Accordingly, the above described flags were required to determine the write-in state of the digital signal, that is, discriminate which part or parts of the one frame of digital signal have been written into the memory circuit. As a result, there was a need to provide a considerable number of addresses in the memory circuit according to this conventional method. Therefore, this conventional method suffered a disadvantage in that the control of the memory circuit became complex, because the operations involved to determine what kind of signal is stored and which address contains such a signal, are quite complex.
In general, the minimum period of the time base fluctuation component included within the digital signal which is reproduced from the disc by the reproducing system, is sufficiently large compared to the frame period. In addition, the time base fluctuation component in the reproduced digital signal is not particularly large. The present inventors derived the present invention by especially noting such facts.